Method for controlling the injection process and injector unit

ABSTRACT

The invention relates to an injector unit and a method for the control of the injection process into the cavities of injection moulding tools. The novel solution permits the injection of thin-walled injection moulding pieces ( 76, 77 ), with both electrically- and hydraulically-driven injector screws. One of the most important features thereof is the active selection of the opening time of the locking system at an optimal pressure in the compression chamber, preferably in the region of the upper half of the compression. The volumetric transfer flow is thus significantly improved in thin-walled injection moulding pieces ( 76, 77 ). Above all the melting pressure in the tool, the flow speed and the useful flow path in the cavities are increased.

TECHNICAL BACKGROUND

[0001] This invention relates to a method of controlling/regulating theinjection stroke and compression stroke for the injection process in thecavities of injection molds, in particular for injection of thin-walledinjection molded articles using an electrically or hydraulically driveninjection screw; this invention also relates to an injection unit, inparticular for producing thin-walled injection molded articles ininjection molds, with a controllable active closure for the nozzleopening and an electrically or hydraulically driven injection screw.

BACKGROUND INFORMATION

[0002] The volume flow transfer behavior from the screw antechamber intothe cavity of the injection mold during the injection phase depends onnumerous parameters and is acknowledged in the technical world as beinga complex process. Results of basic experiments conducted by the presentauthors were published in Plastverarbeiter [Plastics Processor], volume41 (1990). Reference is made below to this publication. Measurementshave shown that there are definite differences between the volumeactually introduced into the cavity and the volume obtained on the basisof geometric data and the forward speed of the screw.

[0003] There have been unsuccessful attempts in the past to calculate anoptimum injection volume flow profile for mold filling by usingprograms. The present authors consider the main factor in thisdifference to be the compressibility of the melt and machinedeformation. Based on our own investigations, it is proposed in thearticle that the deformation, such as that based on the differencebetween the measured total deformation and the calculated meltcompression, first be considered as elastic machine deformation. It hasbeen found that elastic machine deformation is of an order of magnitudesimilar to that of melt compression in the range of low actingpressures. Before performing the measurements, proper functioning of thebackflow valve for the entire molding path was ensured. Nevertheless adifference amounting to a factor of more than two was obtained betweenthe theoretical flow path and the actual flow path. The proposedderivation of the actual flow front position from the theoreticalposition is based on the following simplifications:

[0004] At each discrete point in time in the dynamic injection process,the measured variables are regarded as quasi-static. At each point inthe melt, the same pressure prevails as in the screw antechamber. Thecalculated flow distance was thus slightly below that actually measured.The following reasons are given:

[0005] a) Contrary to the original assumption, different pressuresprevail in the screw antechamber and in the cavity itself.

[0006] b) The pressure is the cavity is not constant over the flow path.It decreases in an approximately linear ratio from the gate to the flowfront.

[0007] c) The volume in the screw antechamber is steadily reduced duringthe injection process.

[0008] d) The plasticated mass was selected to be large in comparisonwith the mass of the molding in order to reduce the errors describedhere.

[0009] These experiments were conducted with final pressures of 800,1100 and 1500 bar, whereby the development of the pressure in thecompression space over time was varied in the range of 20 to 50milliseconds as a function of the metering volume. The rate of increasein volume flow increased with a drop in the final pressure level. Theoutgoing volume flow developed more rapidly accordingly. In theremaining curve the changes in volume flow declined and developed into alinear curve. The transition would occur at an earlier or later point intime, depending on the backpressure level, and at a greater or lesservolume flow level. The changes in volume flow over time increased withan increase in the final pressure level in the linear range. It can thusbe concluded that, when applied to an actual injection molding process,this finding would mean that the volume flow entering the mold wouldshow a linear increase with an increase in the flow length of the meltin the mold after a startup phase. Furthermore, with an increase in thebackpressure end value, the difference in comparison with thetheoretical value of the volume flow would become greater. Thissituation is emphasized as being of particular interest because, basedon practice, filling times have undeniably increased. According to thetechnical publication cited above, an increase in filling time must beexpected in particular when molded parts having small cross sections areunmolded. Developments over the past decade, in somewhat simplifiedterms, have emphasized two trends:

[0010] In the case of injection molding machines using a hydraulicdrive, inexpensive machines have become established for lower qualitydemands and injection molded articles having thick walls. It has beenpossible to meet the highest quality demands only with suitably designedand expensive machines, in particular for injection molded articleshaving thin walls.

[0011] It has, interestingly enough, been impossible to use electricallydriven machines to manufacture thin-walled injection molded articles tomeet the highest demands.

[0012] Production of thin-walled injection molded articles has remaineda specialty for hydraulic machines in the higher price class. Thedynamics of the moving mass forces during injection can be controlledmore easily with these machines.

[0013] WO 01/03906 is based on an injection molding method with whichplastic under pressure is injected from a relatively large antechamber,which can be sealed off, into a mold cavity after opening the gate. Thepurpose was to fill the mold cavity rapidly when producing thin andsmall injection molded articles, where the speed of the injectionplunger should not play any role at all or should play only asubordinate role. The known state of the art is described in this WOpatent as follows:

[0014] “If, even before opening a gate, a pressure comparable to thepressure in the mold interior has been established in front of the gate,this results only in the mold being filled partially at first due toexpansion of the plastic in the antechamber until the effect of theforward feed of the screw is manifested after a slight time lag.”

[0015] WO 01/03906 proposes as an improvement that the pressure in theantechamber be increased to more than 1500 bar, and in particular thatthe volume of the antechamber not be reduced to a great extent, as isconventional in the injection molding operation, but instead that it bemaintained entirely or for the most part. The mold cavity here is filledonly by a process of adiabatic expansion of the plastic material thathas collected in the large antechamber and is under a high pressure. Theadiabatic expansion results in cooling by 30° C. and a drop in pressureto the desired final range of 500 bar. According to one example, theantechamber may be up to 45 times larger in relation to the mold cavity,i.e., 30 to 40 articles can be produced with one antechamber filling.For most applications, a corresponding dwell time of hot plastic melt inthe antechamber and an overheating of 30° C. would not be acceptable.Therefore, this design can be used only in very special cases.

[0016] European Patent 0 513 774 describes another state-of-the-artmethod using a hydraulic drive for stepwise control of a sequence ofinjection cycles with the goal that the dwell pressure always matchesthe dwell pressure of the preceding injection molding cycle. Anadmission pressure is generated between the injection screw and thecasting mold due to the fact that a switching valve is reached by thetime of reaching the pressure corresponding to the previous cycle. Afteropening the switching valve, the pressure collapses during the meteringoperation and is increased again according to the preceding cycle. Thebuildup of the admission pressure and the dwell pressure takes placecompletely independently with the single goal of achieving values as inthe previous cycle. Such a method is not advantageous for thin-walledarticles.

EXPLANATION OF THE INVENTION

[0017] This invention is based on the object of developing a method andan injection unit which will yield an improvement in the injectionprocess, in particular for the production of thin- walled injectionmolded articles, without any restriction on the scope of application.

[0018] The inventive method is characterized in that the compressionstroke is converted without interruption into the injection stroke, andan active closure is opened in a controlled manner at an optimumpressure in the compression space and/or with optimum melt compressionand the transition from melt compression to volume flow transfer isbeing established, the active closure being opened as suddenly aspossible.

[0019] The inventive injection unit is characterized in that it hascontrol/regulating means for a two-step uninterrupted linear movementfor a compression stroke and for an injection stroke of the injectionscrew, whereby the active closure can be operated between thecompression stroke and the injection stroke, and the active closure haselectric or hydraulic drive means, for the most sudden possible openingof the active closure.

[0020] According to an especially advantageous embodiment of thismethod, the opening point in time of the active closure is selected sothat the effect of reflected pressure waves in front of the nozzle hole,i.e., the mold nozzle, is prevented. The present inventor has recognizedthat the highly dynamic process, in particular the startup process, hasbeen investigated in all conceivable directions in the state of the art,but in doing so the active influence on one of the most importantparameters, namely the dynamic response of the closure, has beendisregarded. All previous experiments by the present applicant haveshown that in the case of injection molded articles having thick walls,the necessarily greater complexity for the active closure isquestionable economically because the possible improvements associatedwith it are relatively minor. It has been found that the advantage isgreater, the thinner the walls of the part to be injected. This allowstwo results: First, in the case of hydraulically driven machines, it ispossible to produce thin-walled parts of an extremely high quality byinjection molding with a lower structural complexity and with lesscontrol technology for the entire machine but with a relatively minorincrease in expense for the control of an active closure. Second,thin-walled parts of the highest quality can for the first time beproduced by injection screws driven by electric motor as well as bythose driven with a hydraulic drive. The relevant basic research has sofar been conducted primarily with articles having thicker walls, whichare less problematical, so it was not recognized that an important partof the compression energy immediately after automatic uncontrolledopening of a spring-loaded closure can no longer be converted intovolume velocity by creating a reflected pressure wave in front of thenozzle hole and/or the mold nozzle. In the state of the art, thehydraulic pressure applied in the screw antechamber was converted tocompression only inadequately in the phase of the greatest pressure.This is because the volume flow transfer had already taken place duringthe pressure buildup.

[0021] In the second case according to the citation from WO 01/03906, apositive effect could not be achieved because it was never possible tobring the transition from pressure buildup in the antechamber to thevolume flow transfer under control. WO 01/03906 also controls thetransition by a radical method by not taking into account the effect onthe screw forward feed in the volume flow transfer.

[0022] This novel inventive method proposes the active injection cyclebe performed in two phases, where the compression stroke is the firstphase and the injection stroke is the second phase. The secret of thisnovel method is thus that the two phases are not created by artificialinterruptions in the linear movement of the injection screw but insteadby controlling/opening the active closure in a manner that is controlledas a function of pressure and/or time. This allows the dynamics of themass forces accelerated during the compression stroke to be utilizedmaximally to support the injection stroke. In an exaggerated image, thisnovel method proposes that tension be applied to a compression springwith the greatest possible dynamics, that the front end be released inthe range of the greatest tension and that the entire spring be pushedforward by utilizing the dynamics. Relaxation of the spring then nolonger occurs at the site of tension buildup but instead the relaxationtakes place at a more forward location. When applied to the injectionprocess, it has thus become possible to shift the expansion of thecompressed injection molding material into the mold itself, whichexplains the surprisingly positive effects, namely:

[0023] a better volume flow transfer

[0024] an increase in flow rate

[0025] more rapid filling of the mold

[0026] a long flow path at a higher flow rate

[0027] a great pressure buildup in the mold, especially in the case ofextremely thin-walled articles.

[0028] With this novel inventive method, the energy expended ismaximally converted into compression of the melt for the compressionstroke. Subsequently the compressed melt is transferred to the mold withthe greatest possible kinetic energy due to almost abrupt opening at anoptimum melt pressure, and the resulting maximum speed is utilized forthe volume flow transfer from the screw antechamber into the moldcavities. If the compression energy and/or the expansion of the melt isnot implemented until the melt is inside the mold, this increases themelt velocity in the mold accordingly. Preliminary laboratoryexperiments on very thin-walled articles have already shown that themelt flow rate in the mold is increased significantly with this novelmethod. The melt travels a greater distance in a shorter period of time,which has been confirmed by appropriate sensors for ascertaining theposition of the particular flow front over time. Another essentiallyplausible explanation for the surprising effect of the inventiveembodiment is also that, to put it in negative terms, the effect of thecooled wall parts of the cavity is greater as the walls of the partbecome thinner. If the closure opens passively and usually much toosoon, especially even at a relatively low compression, this leads torapid cooling in the boundary zones at the inlet area of the mold cavityand immediately creates inferior flow conditions for the following meltflow. This novel method makes it possible to largely eliminate thisnegative phenomenon.

[0029] This invention also relates to a number of advantageousembodiments, to which end reference is made to claims 2 through 9 and 11through 16.

[0030] Preferably according to the novel method, a maximum steepness ofthe pressure rise until active opening of the closure system is selectedwithin the range of the allowed operating parameters. In the entirepressure increase phase, the maximum possible compression work is thusexpended. Immediately after opening the closure, both the storedcompression energy and the maximum delivery effect of the injectionscrew on the plastic compression and/or on all accelerated masses isavailable for a maximum of volume flow transfer, and this creates theoptimum startup//oncoming flow conditions in the flow channels asdescribed above. If the injection screw has an electric motor drive or ahydraulic drive and the pressure buildup occurs before opening theactive closure with maximum dynamics, then the highest possible kineticenergy is available at the beginning of the injection if the dynamics ofthe moving parts is maintained without interruption for the injectionprocess. This is equally true for the hydraulic drive and the electricdrive of the injection screw. This novel method opens up a previouslyexcluded area for electrically operated machines. This is especiallyvaluable because electric drives require much less energy per injectionmolded article produced in relation to hydraulic drives. The pressurebuildup in the compression space and the injection process during andimmediately after the opening time of the active closure take placewithout interruption and at maximum motor power. It is important herethat the drive means for the injection screw are selected so that theduration of the pressure buildup is short. The linear drive for theinjection screw may be designed as a servo motor with regulation via thespeed input. The novel method, however, also allows the use of aninexpensive drive motor. According to previous experiments, the bestchoice has been to use a needle valve as the active closure. The activeclosure can then be arranged at the nozzle outlet of the injection screwor in the inlet area of the cavity of the injection mold. This has thegreat advantage for practical use that in many cases this novel methodcan also be implemented by adapting or remodeling molds and without anychanges in the machine.

BRIEF DESCRIPTION OF THE INVENTION

[0031] This novel method will now be explained below with additionaldetails on the basis of a few examples. The figures show:

[0032]FIG. 1: a diagram of the novel method with a needle valve as theactive closure;

[0033]FIG. 2a: an example of a state-of-the-art passive closure with aball gasket and a plate spring;

[0034]FIG. 2b: an example of an active closure designed as a needleclosure according to the novel method;

[0035]FIG. 2c: an example of an active closure with a hydraulicallycontrolled needle according to the novel method;

[0036]FIG. 3: the actively operable needle valve according to FIG. 2b ona larger scale for electric or hydraulic needle closure drives;

[0037]FIG. 4a: an example of an injection mold with which theexperiments were conducted;

[0038]FIG. 4b: the mold part of the experimental injection mold in aview according to section IV-IV, also showing the contour of theinjection molded articles;

[0039]FIG. 5a: schematically one possible explanation for the negativeeffect with respect to the flow of the melt in the state of the art;

[0040]FIG. 5b: the improvement in melt flow rate according to thisinvention;

[0041]FIG. 6: a comparison of active nozzle opening with passive nozzleopening;

[0042]FIGS. 7, 8 and 9: three examples of measurement curves from thebasic experiments for this novel inventive method; FIG. 7: compressionof the melt against a closed nozzle; FIG. 8: injection through a nozzleopening of 1 mm diameter at a theoretical flow rate of 200 mm/sec; andFIG. 9: at a theoretical flow rate of 1000 mm/sec;

[0043]FIG. 10: the most important parameter curves according to thenovel method and the old method;

[0044]FIG. 11: a comparison of the opening times with active and passivevalve control;

[0045]FIG. 12: the time required by the melt A with an active closuresystem and B with a passive closure system to flow from the nozzleantechamber to the most remote sensor.

METHOD AND EMBODIMENT OF THE INVENTION

[0046]FIG. 1 shows schematically an experimental setup for the basicexperiments, whereby the mechanical components are shown only inexcerpts. In an injection cylinder 1 there is an injection screw 2having screw flights 3. For the linear drive of the injection screw 2according to arrow 4, a box M is indicated only schematically withreference number 5. The linear drive may be hydraulic or by electricmotor. A position sensor s-schn is shown with reference number 6 and aflow rate sensor v-schn is shown with reference number 7, likewise onlyschematically. The two sensor values can be obtained from the electroniccontrol unit and/or the corresponding signal generators and computers inthe case of using an electric servo motor. Reference is made to EuropeanPatent 0 647 175 for the control and regulating technology ofelectrically driven injection molding machines. Corresponding controllines and signal lines 8, 9 and 10 lead to a control panel 11. An activeclosure 12 having a needle valve with a valve needle 13 and an operatinglever 14 is operated with a controlled drive (arrow 15). The exact leverposition s-heb is detected by a potentiometer 16 and the command for achange in position is sent from the controller (control panel 11) over asignal line 17 to the corresponding drive, i.e., arrow 15. The letter Kstands for the mold cavity, Esv denotes the injection screw antechamber.The mold cavity K is formed by the two mold halves 18 and 19. For theseexperiments, the melt pressure p_melt was measured directly in thenozzle antechamber DV by a suitable pressure sensor, and the values wererelayed over a charge amplifier 20 to a measurement device 21.Accordingly, the lever position is also reported over a signal line 22to the electronic measurement system of the measurement device 21. Themeasurement device 21 and the control panel 11 are connected to acomputer 23 by control lines and signal lines 26, 27 having a displayscreen 24 and input keyboard 25. In approximate terms, the chronologicalfunction sequence for a casting cycle is as follows: mold closingcompression of melt time delayed opening of the needle · start ofinjection injection/dwell pressure time needle fully open meteredaddition total cycle time

[0047] A central aspect of this new investigation is the separation ofthe command “start injection” and the control command “needle opening”in time and/or the actual injection into the cavity K of the molds 18,19. Suitable programs can be developed and stored for each concretecase, so that the closure system is controlled, i.e., operated activelyat an optimum pressure in the compression space and/or in the nozzleantechamber DV. In this way, the very important random character for theinteraction of the start of injection and the opening of the valve canbe ruled out, which is necessarily the case with all methods usingpassive closures.

[0048] The injection nozzle forms the forward closure for theplastification unit. During the injection process, the nozzle is pressedagainst the gate bushing of the mold. It establishes the connectionbetween the plastifying signal and the gate bushing of the mold for theincoming melt. An injection nozzle should meet the followingrequirements:

[0049] hydraulically favorable design

[0050] easy replaceability

[0051] clean sealing

[0052] retaining the temperature of the melt.

[0053] Previous development efforts have also been concentrated in thedirection of the aforementioned requirements. To prevent material fromescaping during the plastification phase, a wide variety of closurenozzles have been used in the state of the art, especially slidingclosure nozzles, which are usually spring loaded. Spring-loaded slidingclosure nozzles are operated by the action of the pressure of the melt.In many designs, the valve opens already when the nozzle presses againstthe mold.

[0054]FIG. 2a shows as an example a passive closure nozzle having a ballclosure according to the state of the art. A ball 30 pressed by aspecial plate spring 31 having crosswise slots assumes the sealingfunction in this design with respect to the through-opening 32 of thenozzle body 33. The plate spring 31 is held between the nozzle body 33and a nozzle head 34, which has the transfer opening 35 into thecorresponding mold channel opening.

[0055]FIG. 2b shows an example of an active closure. This is a plungernozzle having a radially sealing needle closure with a plunger nozzlehead 40 which is inserted into a mold plate nozzle 40. A heating strip42 ensures that the melt temperature is maintained. The closure needleor valve needle 13 is displaceably mounted in a nozzle insert 43 and ismoved in a controlled manner or held in position by the operating lever14. A device for centering the plunger nozzle head 40 is labeled withreference number 44. The drive of the actuating lever 14 may be ahydraulic or electromechanical drive. It is important for the design toallow rapid movement under high melt pressures.

[0056]FIG. 2c shows schematically a purely hydraulic drive for bothlinear injection screw movement and for active movement of the closureneedle via a hydraulic cylinder 50. The injection screw 2 is moved on alinear path by a hydraulic drive 51. The entire injection unit 52 withthe nozzle head 34 is held by columns 53 and is moved into the injectionposition against a stop by means of another hydraulic drive 54. Athrottle 55 has the function of minimizing strikes against the needlemovement system. Control of needle movement is ensured by a regulatingvalve 56 and a control valve 57.

[0057]FIG. 3 shows the essential components for the closure needle orvalve needle 13 and the operation thereof on a larger scale as aconcrete structural design. The movement of the operating lever 14according to arrow 15 may be accomplished in any desired fashion.Schematically this shows only a power transmission rod 64, which isconnected by an articulated joint 61 to the operating lever 14. Theoperating lever 14 is mounted in a friction-locked mount on theinjection cylinder 1 by way of a rotating pin 62 in a joint head 63,whereby the joint head 63 is detachably mounted by bolts 65 on the frontend of the injection cylinder 1. This fully ensures rapidreplaceability. The melt supply channel 60 is designed with a knee bendhaving channel pieces 60′ and 60″ around the area of the rotating boltand is guided through the ring channel 60′″ around the needlelengthening piece 66 and the valve needle itself. Between the nozzleinsert 43 and the joint head are mounted two transition pieces 67 and 68which connect the plunger nozzle head 40 to the joint head 63 withregard to the transmission of forces. The valve needle 13 is mounted soit is axially displaceable in a bore 69 of the joint head 63 via theneedle lengthening piece 66. The pivoting movement of the actuatinglever 14 necessarily leads to the desired opening and closing movementof the tip of the needle 70 in the corresponding valve seat 71.Insteadof the needle valve shown in FIGS. 2b and 3, an essentially known rotaryslide closure or a snap slide closure may be used.

[0058]FIG. 4a shows a section through a complete injection mold 75,where it can be seen that the intermediate piece 67 is mounted in aplate 73, which is bolted directly to the injection mold. The cavity Kyields two identical injection mold parts 76 and 77, each being L-shapedand being connected at the center by the gate/sprue point 78.A pluralityof measurement points has been arranged in accordance with the publishedarticle cited in the introduction for detection of the entire injectionprocess through measurement technology. For this purpose, eight infraredsensors 79 have been positioned at equidistant positions to record theexact filling operation. In addition, two internal mold pressuresensors, one near the gate and one at a distance from the gate, wereinstalled at the end of the flow path. Each experiments was started withpassive needle opening as the basic machine setting. Then they wereswitched to “active opening.” The following abbreviations are used inthe diagrams in FIGS. 6 and 7:

[0059] Mold wz

[0060] pressure P

[0061] distance S

[0062] plunger Kolb.

[0063] lever position s_heb

[0064] internal mold pressure p_wz

[0065] infrared sensors Front

[0066] screw speed v_sch

[0067] screw position s_sch

[0068] melt pressure nozzle antechamber p_melt

[0069] “Active” means that the closure system is controlled activelyaccording to the novel inventive method, and “passive” means that theclosure system is operated according to the state of the art withoutforced control.

[0070]FIGS. 5a and 5 b show a comparison of the state-of-the-art method(FIG. 5a) with the novel method (FIG. 5b). In FIG. 5a the valve isalready opened at a low melt compression for the startup process. Thisis indicated schematically with the valve being partially opened. Thespeed of the melt flowing forward is relatively low. The edge area 80 iscooled immediately. This results in an artificial narrowing of the canalcross section with a corresponding distorted flow rate profile 81. Thetransfer of volume flow from the screw antechamber to the cavity of theinjection mold has a massively negative effect. FIG. 5b shows anidealized diagram of the situation with the novel method, representedsymbolically with the valve completely opened. Since the maximumpressure, i.e., the maximum compression energy and also the maximumdelivery energy are available suddenly, this yields a broad flow frontwith an almost maximum flow rate V_(max) over the entire cross section.The flow rate profile 82 is fully developed accordingly. The consequenceis a higher flow rate, more rapid filling of the mold and a longer flowpath. Thus long parts having thin walls can be produced by injectionmolding in the shortest possible amount of time. The same thing is alsotrue of the hydraulic drive as well as the electromechanical drive.

[0071]FIG. 6 shows very symbolically a comparison between an activenozzle opening and passive nozzle opening. As expected, the two curvesfor the melt pressure in the screw antechamber active p_melt and passivep_melt directly one above the other during most of the compressionphase. In the active p_melt case, the pressure at the end increases byapprox. 10%. This is also consistent with the expected result because inthe passive case, melt is already beginning to flow out after the startof the compression phase. This is discernible by the forward movement ofthe lever position passive s_heb, where the valve begins to openimmediately. For the full opening distance, the lever requires approx.100 ms from point {circle over (1)} to point {circle over (2)} in thecase of passive opening (without a drive) according to the experimentalexamples. In the active case (with a drive), the opening movement isdelayed with respect to the start of compression, i.e., by approx. 40-50ms, when considered from point {circle over (1)}. The opening time isonly 30-40 ms between points {circle over (3)} and {circle over (4)}.The pressure active p_melt is uniformly as much as 20% below thepressure passive p_melt. The actual surprise lies in the significantpressure difference between the values active p_wz and passive p_wz. Thepressure passive p_wz is approx. 400 bar (0.4 kbar) at point {circleover (5)}, whereas the pressure active p_wz is reached between 500 and550 bar over a long period of time, i.e., at point {circle over (6)},i.e., values more than 25% higher without requiring any additionalexpenditure of energy for the novel inventive method.

[0072]FIG. 7 shows the compression of the melt with the nozzlecompletely closed. This shows the analogy with the novel method, atleast for the pressure curve, during the compression phase. Thecompression distance amounts to 30 mm. The final pressure in thisexample is approx. 1.75 kbar.

[0073]FIGS. 8 and 9 illustrate the basic problem with state-of-the-artmethods. In the example according to FIG. 8, a theoretical flow rate ofthe screw movement of 200 mm/sec was selected, and in the exampleaccording to FIG. 9, a theoretical flow rate of 1000 mm/sec wasselected. FIG. 8 shows that the maximum possible “volume flow rate” is60 mm/sec, and this is at approx. 250 ms. The high screw forward speedof 0-250 ms is practically only converted to melt compression despitethe open nozzle.

[0074]FIG. 9 shows a very interesting effect, marked with a circle inbold. The dotted line curve s_plunger shows a slight reverse movementover a period of time of approximately 50 ms. The screw springs backslightly. The reason for this is to be found in the effect of reflectedwaves, which occur only at higher flow rates. The experimental exampleis at the same time proof that in the state of the art with passivevalve opening, an important part of the compression energy is notconverted into volume flow rate due to the recoiling pressure reflectionwaves. This novel method makes it possible to rule out the very negativeeffect of the reflected waves through controlled opening of the closure,preferably in the area of maximum compression. Volume flow transfer isoptimum in all regards.

[0075]FIG. 10 shows an idealized curve for the screw speed (v-screw)[and] the path of the screw (s-screw). The melt pressure in the screwantechamber (p_melt) and the pressure in the mold (p_mold) [are shown]in particular for the case of an electric motor screw drive. In thediagram shown here, the closure opens (nozzle opening active) afterapproximately one-third of the time. The pressure of the melt in thescrew antechamber remains approximately constant from the point in timeof nozzle opening at maximum pressure. The pressure in the moldincreases steeply from zero at first and remains relatively high for along period of time. This diagram shows that over the time period ofnozzle opening, the speed of the screw (v-screw) and the distancetraveled by the screw (s-screw) remain steady and unchanged over time(t). The compression stroke of the screw develops without interruptioninto the screw filling stroke and/or the injection stroke. The motordrive passes through both ranges without interruption because the timingof active nozzle opening is adapted exactly to the compression and thevolume transfer. As a comparison, the speed of the screw in passiveopening of the closure is shown with dotted lines, corresponding to thedistance traveled by the screw (passive) for the traditional state ofthe art in the hydraulic drive of the injection screw. In passiveopening of the nozzle closure, the injection screw is accelerated beyondthe required extent at the beginning of the injection cycle so that thespeed drops to half for the volume flow transfer.

[0076]FIGS. 11a and 12 b show a comparison of active and passive closuresystem, where A denotes active opening and B denotes passive opening.The time in milliseconds indicates how much time the melt takes to flowfrom the nozzle antechamber to the last sensor. In the active closuresystem of the novel method, 82.8 ms less time (52%) is needed incomparison with the state-of-the-art passive closure system for the meltto travel the same distance.

1. A method of controlling the injection stroke and the compressionstroke for the process of injection into the cavities of injectionmolds, in particular for production of thin-walled injection moldedarticles (76, 77) using an electrically or hydraulically driveninjection screw, characterized in that the compression stroke developsinto the injection stroke without interruption, and an active closure(12) is opened in a controlled manner at an optimum pressure in thecompression space and/or at optimum melt compression, and the transitionfrom melt compression to volume flow transfer is established, with theactive closure being opened as rapidly as possible.
 2. The methodaccording to claim 1, characterized in that the slide opening takesplace in the upper half of the maximum specific melt pressure in thescrew antechamber.
 3. The method according to claim 1 or 2,characterized in that the opening point in time of the active closure(12) is preferably selected so that expansion of the compressed melt andconversion to flow front velocity take place primarily in the cavitiesof the mold.
 4. The method according to one of claims 1 through 3,characterized in that the energy of the moving parts of the rotatoryand/or translational masses accelerated during the compression stroke isutilized maximally to support the injection stroke.
 5. The methodaccording to one of claims 1 through 4, characterized in that the driveof the injection screw (2) is provided by an electric motor and thepressure buildup in the screw antechamber before opening the activeclosure (12) takes place in such a way that the moving parts of thedrive unit have the greatest possible kinetic energy at the start of theinjection (active closure opening).
 6. The method according to one ofclaims 1 through 5, characterized in that the rotational speed of theelectric drive motor of the injection screw in the opening of the slideis in the range of he maximum motor rotational speed, so that the energyof the moving parts of the rotatory and/or translational massesaccelerated during the compression stroke is used to support theinjection stroke maximally.
 7. The method according to one of claims 1through 6, characterized in that the compression stroke for eachinjection cycle amounts to up to 40% of the total screw stroke.
 8. Themethod according to one of claims 1 through 7, characterized in thatduring and/or after opening of the nozzle, the speed of movement of theinjection screw is maintained to prevent a drop in pressure in the screwantechamber.
 9. The method according to one of claims 1 through 8,characterized in that the pressure buildup in the compression space, atleast before the opening point in time of the active closure (12), iscontinued at the maximum driving power and/or motor power, and theinjection process, at least directly after the opening point in time, isalso continued at the maximum driving power and/or motor power.
 10. Aninjection unit in particular for injection of thin-walled injectionmolded articles (76, 77) in injection molds having a controllable activeclosure (12) for the nozzle opening and an electrically or hydraulicallydriven injection screw (2), characterized in that it hascontrol/regulating means for a two-step uninterrupted linear movement,for a compression stroke and for an injection stroke of the injectionscrew (2), whereby the active closure (12) can be operated between thecompression stroke and the injection stroke, whereby the active closurehas electric or hydraulic drive means for the most sudden possibleopening of the active closure.
 11. The injection unit according to claim10, characterized in that the control and regulating means as well asthe active closure are designed so that the slide opening isimplementable in the upper half, preferably in the upper fourth of therange of buildup of the specific melt pressure in the screw antechamber.12. The injection unit according to claim 11, characterized in that theactive closure (12) has electric or hydraulic drive means for the mostsudden possible opening of the nozzle, preferably with the openingprocess lasting less than 50 milliseconds.
 13. The injection unitaccording to one of claims 10 through 12, characterized in that theactive closure (12) is arranged on the nozzle outlet of the injectionscrew (2) or in the inlet area of the cavity of the injection mold, andit is designed as a needle valve, a plug slide valve or a rotary slidevalve.
 14. The injection unit according to one of claims 10 through 13,characterized in that a corresponding total screw stroke can beimplemented cyclically with each injection cycle in accordance with thesize of the injection mold part (76, 77), only a small cushion of meltremaining at the end of the injection stroke.
 15. The injection unitaccording to one of claims 10 through 14, characterized in that thelinear drive for the injection screw (2) has an electric motor, theelectric motor being designed with electronic power systems with therespective computer means and control/regulating means (11).
 16. Theinjection unit according to one of claims 10 through 15, characterizedin that the program memories and computers (23) are assigned to theinjection unit for cyclic coordination of the active opening of theclosure such that the full opening of the active closure (12)corresponds approximately to the maximum compression of the melt.